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Single standard calibration for an optical oxygen sensor based on luminescence quenching of a ruthenium complex
Analytica Chimica Acta 403 (2000) 57–65
Single standard calibration for an optical oxygen sensor based on luminescence quenching of a ruthenium complex Martin M.F. Choi ∗ , Dan Xiao1 Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China Received 16 January 1999; accepted 3 August 1999
Abstract An optical oxygen (O2 ) sensor consisting tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) ditetrakis(4-chlorophenyl) borate adsorbed on silica gel has been successfully fabricated and used to continuously monitor O2 gas at low concentration. The luminescence material shows a very strong and stable pink emission when excited by blue light and it is efficiently quenched by O2 . The calibration of the optical sensor can be simply done by a single standard flow injection method. The results demonstrate excellent linear Stern–Volmer behaviour when O2 concentration is at low levels (0.0–0.05% v/v), but the Stern–Volmer plot has a slightly downward curvature at higher O2 levels. The two-site quenching model correlates well with the calibrated O2 concentration range (0.0–0.55%). The t95 response times of the sensor are <0.2 s on going from 0.0% to 0.55% O2 and <1 s on going from 0.55% to 0.0% O2 . The sensor has high photostability, a long lifetime and no hysteresis in the response. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium(II) complex; Oxygen; Optical sensor; Single standard calibration
1. Introduction Oxygen (O2 ) is considered to be one of the most important gases in our environment since it is found as either reactants or products in a vast number of chemical and biochemical reactions. Considerable efforts have been devoted to the development of optical and electrochemical methods for the quantitation of O2 . To date the Clark electrode or its modifications [1], which is based on the electro-reduction of O2 on a polarized cathode, is one of the most widely used ∗ Corresponding author. Tel.: +852-2339-7839; fax: +852-23397348 E-mail address: [email protected] (M.M.F. Choi) 1 Visiting scholar on leave from the College of Chemistry and Chemical Engineering, Hunan University, Changsha, P.R. China.
O2 detectors. Although the Clark electrode functions well in most situations, it also has some limitations including the consumption of O2 , relatively long response times and the tendency of the electrode to become poisoned by contaminants such as proteins, hydrogen sulphide and organic compounds. Therefore, a tremendous effort has been invested in developing optical sensors for O2 measurements over the past three decades [2–37]. The advantages of these sensors are basically: no O2 consumption in the sensing process, no requirement for a reference electrode, inertness against sample flow rate or stirring speed and immunity to exterior electromagnetic field interference. Ruthenium(II) complexes are by far the most widely used O2 indicator because these metal complexes, in general, have efficient luminescences, relatively long-life metal-to-ligand charge-transfer excited
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states, fast response times, strong visible absorptions, large Stokes shifts and high photochemical stability; these factors account for their widespread popularity in quenching-based O2 sensors. Furthermore, their long excitation and emission wavelengths are more compatible with current solid state opto-electronics monitoring technology [38,39]. Ruthenium(II) complexes adsorbed onto silica gel particles have been successfully employed for fabrication of O2 sensors [40–43]. Silica gel is a useful solid substrate for optical O2 sensors because of its high thermal stability, good photostability and optical transparency in the visible region. It is anticipated that the use of silica gel-adsorbed ruthenium(II) complex will provide an attractive approach for optical O2 sensing. The calibration of most luminescence quenchingbased optical sensors in essence relies on the well known Stern–Volmer equation [44] : I0 = 1 + kSV [O2 ] I
graphs for the Stern–Volmer plots. In principle, a single standard injection in conjunction with the exponential dilution method should be able to calibrate the O2 sensors. In this paper, the development of an optical O2 sensor based on the luminescence quenching of tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) ditetrakis(4-chlorophenyl)borate, [Ru(dpp)3 ][(4-Clph)4 B]2 -adsorbed silica gel packed in a flow cell is reported. The Ru(II) complex-adsorbed silica gel shows a very strong and stable pink emission when excited by blue light and it is efficiently quenched by O2 . A single standard injection method incorporating the exponential dilution technique is employed to calibrate the optical O2 sensor. This proposed method is successfully applied to demonstrate the Stern–Volmer relationship of the sensor at low concentrations and it also relates well to the two-site quenching model [45] from low to high O2 concentration levels.
(1)
where I0 and I are the luminescence intensities in the absence and presence of O2 , respectively, [O2 ] is the O2 concentration and kSV is the Stern–Volmer quenching constant. Very often a wide range of standard gases varying from low to high concentrations are required to fully characterize the Stern–Volmer behaviour of quenching-based optical sensors. However, the generation of gaseous standards usually demands a gas mixing system consisting of a stream of diluent gas mixed with a primary stream of standard gas in a well defined proportion. The set-up of such a gas mixing system at least requires two or more mass flow controllers and a gas mixing chamber. In addition, the concentration range obtained from this continuous flow system is also limited by the concentration of the primary stream applied to the gas mixing system. From our experience, it is inconvenient to use the gas mixing system to generate a lot of gaseous standards. Difficulty will also be encountered under the circumstance that low concentration O2 standards are needed to calibrate some extremely O2 -sensitive optical sensors. To the best of our knowledge, several articles have already adopted systems that can calibrate optical O2 sensors at the near parts-per-million (ppm) level based on the flow injection and exponential dilution method [24,32,36]. Unfortunately, these techniques still require multiple standards injection in order to set up the calibration
2. Experimental 2.1. Chemicals and reagents Acetone, diethyl ether, N,N-dimethylformamide (DMF), 4,7-diphenyl-1,10-phenanthroline, ethanol, ethylene glycol, ruthenium(III) chloride hydrate, sodium chloride, tetrahydrofuran (THF) and toluene were purchased from Aldrich (Milwaukee, WI). Potassium tetrakis(4-chlorophenyl)borate was obtained from Fluka Chemicals (Buchs, Switzerland). The dye ion-pair, [Ru(dpp)3 ][(4-Clph)4 B]2 , was synthesized according to a modified method [23]. All chemicals were used as received. [Ru(dpp)3 ][(4-Clph)4 B]2 adsorbed silica gel was synthesized by the following procedure: 10 g of silica gel 60 (Merck, Darmstadt, Germany) was stirred overnight with 10 ml of a 0.1 M ethanolic solution of [Ru(dpp)3 ][(4-Clph)4 B]2 . The solid residue of [Ru(dpp)3 ][(4-Clph)4 B]2 -adsorbed silica gel was filtered and washed successively with THF, diethyl ether, acetone, ethanol and water in order to remove excess and unadsorbed [Ru(dpp)3 ][(4-Clph)4 B]2 dye ion-pair. This solid residue was put in an oven at 100◦ C for 5 h to evaporate the solvents. The dry [Ru(dpp)3 ][(4-Clph)4 B]2 -
M.M.F. Choi, D. Xiao / Analytica Chimica Acta 403 (2000) 57–65
adsorbed silica gel was kept in a desiccator for further use.
2.2. Instrumentation The [Ru(dpp)3 ][(4-Clph)4 B]2 -adsorbed silica gel was packed in a specially designed home-made flow-through cell shown in Fig. 1 and positioned in a spectrofluorimeter consisting of a lamp power supply (Model LPS-220), a xenon lamp (Model A1010) and a photomultiplier detection system (Model 710), from Photon Technology International (Ont., Canada). Different concentrations of O2 standards were injected into the flow-through cell. All fluorescence measurements were made at room temperature of (23◦ C) and atmospheric pressure.
2.3. Gas flow systems Two gas flow systems were used to introduce O2 standards into the flow-through cell. Fig. 2 illustrates a continuous injection mode of gaseous O2 standards. A stream of N2 (Chun Wang Industrial Gases, Shenzhen, China) was mixed with a stream of 1% (v/v) O2 balanced by N2 (Hong Kong Oxygen & Acetylene Co. Ltd.) in a well defined proportion in a gas mixing chamber. A steady environment of N2 –O2 gas mixtures of various specified compositions, spanning the range 0.0–1.0% (v/v) O2 , were generated using two mass flow controllers (Brooks Instrument B.V., The Netherlands). The total flow rate was kept constant at 100 ml min−1 . Fig. 3 shows the flow injection of an air (Chun Wang Industrial Gases, Shenzhen, China) sample into the flow-through cell. An air sample (0.2–0.5 ml) was introduced into an exponential dilution bottle (135 ml) with a Hamilton gas-tight syringe through a rubber stopper. The exponential dilution bottle was filled with argon (Hong Kong Special Gas Co.) and contained a magnetic stirrer to produce gas homogeneity. The exponential dilution bottle was continuously flushed with a steady stream of argon (Ar) carrier gas at a flow rate of 110 ml min−1 and dragged the injected air sample to the flow-through cell. In this way, the initial concentration, C0 , in the bottle is exponentially diluted according to the expression [46].
Qt C = C0 exp − V
59
(2)
where C is the concentration at time t, Q is the volumetric flow rate of the Ar carrier gas, V is the effective volume of the exponential dilution bottle and t is the time after introducing the sample. Clearly, this equation relates C to t exponentially. By recording t we can obtain the concentration of O2 reaching the flow-through cell at time t.
3. Results and discussion 3.1. Spectroscopic behaviour of oxygen-sensitive material The Ru(II) complex-adsorbed silica gel shows a very strong and stable pink emission when excited by blue light and it is efficiently quenched by molecular O2 . The fluorescence emission spectra of the O2 -sensitive material in contact with different concentrations of O2 generated by the continuous injection mode are shown in Fig. 4. The luminescence material displays strong fluorescence at 604 nm when it is excited at 468 nm. The fluorescence intensity at 604 nm decreases with increasing concentration of O2 . It can be seen that the fluorescence emission is severely quenched by O2 even though the O2 concentration is as low as 0.05–1.0% (v/v). It is anticipated that this O2 sensor should be sensitive enough to detect O2 at levels below 0.05% (v/v). Unfortunately, the back-diffusion process in the mixing chamber limits the use of the continuous flow system to generate N2 –O2 standards below 0.05% (v/v). Furthermore, it is costly to purchase another O2 standard at a low concentration level for continuous gas mixing. It is also time-consuming and troublesome to use the gas mixing system to generate a lot of different gaseous standards. 3.2. Single standard calibration In order to circumvent the above drawbacks, a flow injection and exponential dilution technique was investigated to allow single standard calibration. Air (20.95% v/v O2 ) sample at volumes of 0.2–0.5 ml were injected into the exponential dilution bottle shown in
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Fig. 1. Schematic diagram of the home-made flow-through cell. (1) stainless steel support with sample inlet and outlet (2) fixing screw; (3) stainless steel cover; (4) plastic O-ring seal; (5) quartz glass support; (6) [Ru(dpp)3 ][(4-Clph)4 B]2 -adsorbed silica gel particles; cell volume is 0.38 ml; (7) cotton wool.
Fig. 2. Schematic diagram of the continuous injection mode gas mixing system for generation of oxygen standards.
Fig. 3. Schematic diagram of the flow injection and exponential dilution system for generation of oxygen standards.
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Fig. 4. Fluorescence emission spectra of the oxygen-sensitive material when subjected to various oxygen concentrations using the continuous injection mode. Excitation wavelength is 468 nm. (1) 0.0; (2) 0.05; (3) 0.10; (4) 0.15; (5) 0.20; (6) 0.25; (7) 0.30; (8) 0.35; (9) 0.40; (10) 0.45; (11) 0.50; (12) 0.55; (13) 1.0% (v/v) O2 .
Fig. 3. The fluorescence emission intensity was then monitored against time (Fig. 5). It is observed that the emission intensities decrease dramatically first and recover their intensities afterwards. This flow injection system gave rise to the formation of a sample (O2 ) plug which reached the flow-through cell with an exponential decay concentration profile illustrated in Eq. (2). By recording t, the concentration of O2 gas reaching the flow-through cell at time t can be easily obtained. The quenching-based optical sensor can be calibrated by the Stern–Volmer equation shown in Eq. (1). The relative emission intensity ratios (I0 /I) − 1 against the O2 concentration employing the flow injection and exponential dilution system were plotted. The statistical results of the calibration graphs are displayed in Table 1. The Stern–Volmer plots of the calibration graphs showed excellent linearity and there was not much difference in calibration for injecting air sample with volumes varying from 0.2 to 0.5 ml. In addition, an open air sample from our laboratory also gave sim-
ilar results. The remarkable advantages of this method are that a single air standard from open air is adequate to calibrate the O2 sensor and there is no need to use standard O2 gases. It is very simple to measure O2 gas even at trace levels (ppm range).
3.3. Two-site quenching model The flow injection and exponential dilution method can calibrate the O2 sensor at a low concentration range (0–0.05% v/v). However, it was observed that there was a downward curvature of the Stern–Volmer plots especially at higher O2 concentrations. When the luminescent transition metal complex is immobilized in a homogeneous microchemical environment, it is well known that the luminescence intensities from the excited state of the transition metal complexes are quenched by O2 according to the Stern–Volmer equation. But in the case of a heterogeneous envi-
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Fig. 5. The fluorescence emission intensity of the oxygen-sensitive material monitored against time using the flow injection and exponential dilution system. Air (20.95% v/v O2 ) sample at volumes of (1) 0.2; (2) 0.3; (3) 0.4; (4) 0.5 ml were injected into the system. Table 1 Linear regression analysis for oxygen standards employing flow injection and exponential dilution system Air injection volume (ml)
Slope (%−1 )
Y-intercept
r2
n
0.2 0.3 0.4 0.5
12.06 ± 0.09 12.18 ± 0.08 11.84 ± 0.06 11.64 ± 0.05
−0.00654 ± 0.00110 −0.00702 ± 0.00136 −0.01146 ± 0.00142 −0.01078 ± 0.00134
0.9990 0.9992 0.9994 0.9995
21 23 25 28
ronment the Stern–Volmer plots of the O2 quenching data for luminescence transition metal complexes will turn nonlinear at high concentrations of O2 [8,12,15]. The nonlinearities of the Stern–Volmer plots can be attributed to the existence of inhomogeneities in the binding sites in the silica gel leading to the differences in the local O2 quenching constants. The curvature of the Stern–Volmer plot is a typical characteristic of multiple species. In general, a two-site quenching model is sufficient to describe the quenching behaviour of O2 on the ruthenium complex immobilized on the substrate. The two-site Stern–Volmer equation [8,47,48] is
I0 = I
f1 1 + kSV1 [O2 ]
+
f2 1 + kSV2 [O2 ]
−1 (3)
where I0 and I are the luminescence intensities in the absence and presence of O2 , respectively, [O2 ] is the O2 concentration, f1 and f2 are the fractions of the quenching sites 1 and 2, respectively; kSV1 and kSV2 are the Stern–Volmer quenching constants for sites 1 and 2, respectively. This nonlinear two-site Stern–Volmer equation was then employed to fit the data sets obtained from the continuous flow and flow injection methods so as to cover a wide range of O2 standards from low to high concentrations. Fig. 6 dis-
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63
Fig. 6. The calibration plot for the oxygen-sensitive material at various oxygen contents using the two-site quenching model. Stern–Volmer plot of (I0 /I) versus O2 concentration. The inset diagram shows the linear regression plot at low O2 concentration ranges (0.0–0.056% v/v).
Table 2 Two-site fitting parameters for quenching of the oxygen-sensitive material O2 concentration range (%)
f1
f2
kSV1
kSV2
r2
n
0.0−0.55
0.54 ± 0.08
0.46 ± 0.08
4.94 ± 0.53
22.46 ± 4.05
0.9997
36
plays the two-site Stern–Volmer plot, and the fitting parameters for the two-site model are also given in Table 2. The data sets fit extremely well to Eq. (3). The fractions of quenching sites 1 and 2 are approximately equal but their quenching efficiencies are completely different. Quenching site 2 is about 4.5 times more efficient than quenching site 1. The inset diagram shown in Fig. 6 indicates that the calibration graph is linear at low O2 concentrations (0.0–0.056% v/v). At low O2 concentrations Eq. (3) is simply converted to Eq. (1) according to the fitting parameters for the two-site model (Table 2). The limit of detection calculated from Eq. (1) as the O2 concentration which produced an analytical signal equal to three times the standard deviation of (I0 /I) − 1 at zero value was found to be 3.6 ppm (v/v).
3.4. Reversibility and response time Fig. 7 shows the typical response of the O2 -sensitive material towards step changes of various O2 concentrations. It demonstrates that the sensor can continuously monitor the O2 content. The response is fully reversible and no hysteresis is found. The t95 response times of the sensor are <0.2 s on going from N2 to 1.0% O2 and <1 s on going from 1.0% O2 to N2 . 3.5. Photostability and long-term stability The photostability of the O2 -sensitive material is extremely good. There was no sign of photodegradation when it was irradiated at 468 nm using an xenon
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Fig. 7. Typical response curves of oxygen-sensitive material at excitation/emission wavelengths of 468/604 nm on exposure to various O2 contents (0.0–0.55% v/v) using the continuous injection mode.
lamp which was set at 70 W for 12 h. The fluorescence intensity was stable throughout 12 h excitation light exposure. The long-term stability of the optical sensor is also good since the analytical performance of the optical sensor did not show any change over 6 months storage in the dry box.
4. Conclusions A single standard calibration method has been successfully applied to calibrate the O2 -sensitive material tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) ditetrakis(4-chlorophenyl)borate-adsorbed silica gel which is capable of monitoring O2 content continuously. The ruthenium(II) complex-adsorbed silica gel possesses high photochemical stability and is not leached out by washing with organic solvents [49] or aqueous solutions. The sensor has high photostability, no hysteresis and long lifetime. Last but not the least the optical sensor can also be easily adapted for
remote measurements by using fibre optics in future developments.
Acknowledgements Financial support of this research from HKBU (project nos.: FRG/96-97/II-54 and FRG/96-97/II-62) is gratefully acknowledged.
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Single standard calibration for an optical oxygen sensor based on luminescence quenching of a ruthenium complex Martin M.F. Choi ∗ , Dan Xiao1 Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China Received 16 January 1999; accepted 3 August 1999
Abstract An optical oxygen (O2 ) sensor consisting tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) ditetrakis(4-chlorophenyl) borate adsorbed on silica gel has been successfully fabricated and used to continuously monitor O2 gas at low concentration. The luminescence material shows a very strong and stable pink emission when excited by blue light and it is efficiently quenched by O2 . The calibration of the optical sensor can be simply done by a single standard flow injection method. The results demonstrate excellent linear Stern–Volmer behaviour when O2 concentration is at low levels (0.0–0.05% v/v), but the Stern–Volmer plot has a slightly downward curvature at higher O2 levels. The two-site quenching model correlates well with the calibrated O2 concentration range (0.0–0.55%). The t95 response times of the sensor are <0.2 s on going from 0.0% to 0.55% O2 and <1 s on going from 0.55% to 0.0% O2 . The sensor has high photostability, a long lifetime and no hysteresis in the response. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium(II) complex; Oxygen; Optical sensor; Single standard calibration
1. Introduction Oxygen (O2 ) is considered to be one of the most important gases in our environment since it is found as either reactants or products in a vast number of chemical and biochemical reactions. Considerable efforts have been devoted to the development of optical and electrochemical methods for the quantitation of O2 . To date the Clark electrode or its modifications [1], which is based on the electro-reduction of O2 on a polarized cathode, is one of the most widely used ∗ Corresponding author. Tel.: +852-2339-7839; fax: +852-23397348 E-mail address: [email protected] (M.M.F. Choi) 1 Visiting scholar on leave from the College of Chemistry and Chemical Engineering, Hunan University, Changsha, P.R. China.
O2 detectors. Although the Clark electrode functions well in most situations, it also has some limitations including the consumption of O2 , relatively long response times and the tendency of the electrode to become poisoned by contaminants such as proteins, hydrogen sulphide and organic compounds. Therefore, a tremendous effort has been invested in developing optical sensors for O2 measurements over the past three decades [2–37]. The advantages of these sensors are basically: no O2 consumption in the sensing process, no requirement for a reference electrode, inertness against sample flow rate or stirring speed and immunity to exterior electromagnetic field interference. Ruthenium(II) complexes are by far the most widely used O2 indicator because these metal complexes, in general, have efficient luminescences, relatively long-life metal-to-ligand charge-transfer excited
0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 6 4 0 - 6
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states, fast response times, strong visible absorptions, large Stokes shifts and high photochemical stability; these factors account for their widespread popularity in quenching-based O2 sensors. Furthermore, their long excitation and emission wavelengths are more compatible with current solid state opto-electronics monitoring technology [38,39]. Ruthenium(II) complexes adsorbed onto silica gel particles have been successfully employed for fabrication of O2 sensors [40–43]. Silica gel is a useful solid substrate for optical O2 sensors because of its high thermal stability, good photostability and optical transparency in the visible region. It is anticipated that the use of silica gel-adsorbed ruthenium(II) complex will provide an attractive approach for optical O2 sensing. The calibration of most luminescence quenchingbased optical sensors in essence relies on the well known Stern–Volmer equation [44] : I0 = 1 + kSV [O2 ] I
graphs for the Stern–Volmer plots. In principle, a single standard injection in conjunction with the exponential dilution method should be able to calibrate the O2 sensors. In this paper, the development of an optical O2 sensor based on the luminescence quenching of tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) ditetrakis(4-chlorophenyl)borate, [Ru(dpp)3 ][(4-Clph)4 B]2 -adsorbed silica gel packed in a flow cell is reported. The Ru(II) complex-adsorbed silica gel shows a very strong and stable pink emission when excited by blue light and it is efficiently quenched by O2 . A single standard injection method incorporating the exponential dilution technique is employed to calibrate the optical O2 sensor. This proposed method is successfully applied to demonstrate the Stern–Volmer relationship of the sensor at low concentrations and it also relates well to the two-site quenching model [45] from low to high O2 concentration levels.
(1)
where I0 and I are the luminescence intensities in the absence and presence of O2 , respectively, [O2 ] is the O2 concentration and kSV is the Stern–Volmer quenching constant. Very often a wide range of standard gases varying from low to high concentrations are required to fully characterize the Stern–Volmer behaviour of quenching-based optical sensors. However, the generation of gaseous standards usually demands a gas mixing system consisting of a stream of diluent gas mixed with a primary stream of standard gas in a well defined proportion. The set-up of such a gas mixing system at least requires two or more mass flow controllers and a gas mixing chamber. In addition, the concentration range obtained from this continuous flow system is also limited by the concentration of the primary stream applied to the gas mixing system. From our experience, it is inconvenient to use the gas mixing system to generate a lot of gaseous standards. Difficulty will also be encountered under the circumstance that low concentration O2 standards are needed to calibrate some extremely O2 -sensitive optical sensors. To the best of our knowledge, several articles have already adopted systems that can calibrate optical O2 sensors at the near parts-per-million (ppm) level based on the flow injection and exponential dilution method [24,32,36]. Unfortunately, these techniques still require multiple standards injection in order to set up the calibration
2. Experimental 2.1. Chemicals and reagents Acetone, diethyl ether, N,N-dimethylformamide (DMF), 4,7-diphenyl-1,10-phenanthroline, ethanol, ethylene glycol, ruthenium(III) chloride hydrate, sodium chloride, tetrahydrofuran (THF) and toluene were purchased from Aldrich (Milwaukee, WI). Potassium tetrakis(4-chlorophenyl)borate was obtained from Fluka Chemicals (Buchs, Switzerland). The dye ion-pair, [Ru(dpp)3 ][(4-Clph)4 B]2 , was synthesized according to a modified method [23]. All chemicals were used as received. [Ru(dpp)3 ][(4-Clph)4 B]2 adsorbed silica gel was synthesized by the following procedure: 10 g of silica gel 60 (Merck, Darmstadt, Germany) was stirred overnight with 10 ml of a 0.1 M ethanolic solution of [Ru(dpp)3 ][(4-Clph)4 B]2 . The solid residue of [Ru(dpp)3 ][(4-Clph)4 B]2 -adsorbed silica gel was filtered and washed successively with THF, diethyl ether, acetone, ethanol and water in order to remove excess and unadsorbed [Ru(dpp)3 ][(4-Clph)4 B]2 dye ion-pair. This solid residue was put in an oven at 100◦ C for 5 h to evaporate the solvents. The dry [Ru(dpp)3 ][(4-Clph)4 B]2 -
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adsorbed silica gel was kept in a desiccator for further use.
2.2. Instrumentation The [Ru(dpp)3 ][(4-Clph)4 B]2 -adsorbed silica gel was packed in a specially designed home-made flow-through cell shown in Fig. 1 and positioned in a spectrofluorimeter consisting of a lamp power supply (Model LPS-220), a xenon lamp (Model A1010) and a photomultiplier detection system (Model 710), from Photon Technology International (Ont., Canada). Different concentrations of O2 standards were injected into the flow-through cell. All fluorescence measurements were made at room temperature of (23◦ C) and atmospheric pressure.
2.3. Gas flow systems Two gas flow systems were used to introduce O2 standards into the flow-through cell. Fig. 2 illustrates a continuous injection mode of gaseous O2 standards. A stream of N2 (Chun Wang Industrial Gases, Shenzhen, China) was mixed with a stream of 1% (v/v) O2 balanced by N2 (Hong Kong Oxygen & Acetylene Co. Ltd.) in a well defined proportion in a gas mixing chamber. A steady environment of N2 –O2 gas mixtures of various specified compositions, spanning the range 0.0–1.0% (v/v) O2 , were generated using two mass flow controllers (Brooks Instrument B.V., The Netherlands). The total flow rate was kept constant at 100 ml min−1 . Fig. 3 shows the flow injection of an air (Chun Wang Industrial Gases, Shenzhen, China) sample into the flow-through cell. An air sample (0.2–0.5 ml) was introduced into an exponential dilution bottle (135 ml) with a Hamilton gas-tight syringe through a rubber stopper. The exponential dilution bottle was filled with argon (Hong Kong Special Gas Co.) and contained a magnetic stirrer to produce gas homogeneity. The exponential dilution bottle was continuously flushed with a steady stream of argon (Ar) carrier gas at a flow rate of 110 ml min−1 and dragged the injected air sample to the flow-through cell. In this way, the initial concentration, C0 , in the bottle is exponentially diluted according to the expression [46].
Qt C = C0 exp − V
59
(2)
where C is the concentration at time t, Q is the volumetric flow rate of the Ar carrier gas, V is the effective volume of the exponential dilution bottle and t is the time after introducing the sample. Clearly, this equation relates C to t exponentially. By recording t we can obtain the concentration of O2 reaching the flow-through cell at time t.
3. Results and discussion 3.1. Spectroscopic behaviour of oxygen-sensitive material The Ru(II) complex-adsorbed silica gel shows a very strong and stable pink emission when excited by blue light and it is efficiently quenched by molecular O2 . The fluorescence emission spectra of the O2 -sensitive material in contact with different concentrations of O2 generated by the continuous injection mode are shown in Fig. 4. The luminescence material displays strong fluorescence at 604 nm when it is excited at 468 nm. The fluorescence intensity at 604 nm decreases with increasing concentration of O2 . It can be seen that the fluorescence emission is severely quenched by O2 even though the O2 concentration is as low as 0.05–1.0% (v/v). It is anticipated that this O2 sensor should be sensitive enough to detect O2 at levels below 0.05% (v/v). Unfortunately, the back-diffusion process in the mixing chamber limits the use of the continuous flow system to generate N2 –O2 standards below 0.05% (v/v). Furthermore, it is costly to purchase another O2 standard at a low concentration level for continuous gas mixing. It is also time-consuming and troublesome to use the gas mixing system to generate a lot of different gaseous standards. 3.2. Single standard calibration In order to circumvent the above drawbacks, a flow injection and exponential dilution technique was investigated to allow single standard calibration. Air (20.95% v/v O2 ) sample at volumes of 0.2–0.5 ml were injected into the exponential dilution bottle shown in
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Fig. 1. Schematic diagram of the home-made flow-through cell. (1) stainless steel support with sample inlet and outlet (2) fixing screw; (3) stainless steel cover; (4) plastic O-ring seal; (5) quartz glass support; (6) [Ru(dpp)3 ][(4-Clph)4 B]2 -adsorbed silica gel particles; cell volume is 0.38 ml; (7) cotton wool.
Fig. 2. Schematic diagram of the continuous injection mode gas mixing system for generation of oxygen standards.
Fig. 3. Schematic diagram of the flow injection and exponential dilution system for generation of oxygen standards.
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Fig. 4. Fluorescence emission spectra of the oxygen-sensitive material when subjected to various oxygen concentrations using the continuous injection mode. Excitation wavelength is 468 nm. (1) 0.0; (2) 0.05; (3) 0.10; (4) 0.15; (5) 0.20; (6) 0.25; (7) 0.30; (8) 0.35; (9) 0.40; (10) 0.45; (11) 0.50; (12) 0.55; (13) 1.0% (v/v) O2 .
Fig. 3. The fluorescence emission intensity was then monitored against time (Fig. 5). It is observed that the emission intensities decrease dramatically first and recover their intensities afterwards. This flow injection system gave rise to the formation of a sample (O2 ) plug which reached the flow-through cell with an exponential decay concentration profile illustrated in Eq. (2). By recording t, the concentration of O2 gas reaching the flow-through cell at time t can be easily obtained. The quenching-based optical sensor can be calibrated by the Stern–Volmer equation shown in Eq. (1). The relative emission intensity ratios (I0 /I) − 1 against the O2 concentration employing the flow injection and exponential dilution system were plotted. The statistical results of the calibration graphs are displayed in Table 1. The Stern–Volmer plots of the calibration graphs showed excellent linearity and there was not much difference in calibration for injecting air sample with volumes varying from 0.2 to 0.5 ml. In addition, an open air sample from our laboratory also gave sim-
ilar results. The remarkable advantages of this method are that a single air standard from open air is adequate to calibrate the O2 sensor and there is no need to use standard O2 gases. It is very simple to measure O2 gas even at trace levels (ppm range).
3.3. Two-site quenching model The flow injection and exponential dilution method can calibrate the O2 sensor at a low concentration range (0–0.05% v/v). However, it was observed that there was a downward curvature of the Stern–Volmer plots especially at higher O2 concentrations. When the luminescent transition metal complex is immobilized in a homogeneous microchemical environment, it is well known that the luminescence intensities from the excited state of the transition metal complexes are quenched by O2 according to the Stern–Volmer equation. But in the case of a heterogeneous envi-
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Fig. 5. The fluorescence emission intensity of the oxygen-sensitive material monitored against time using the flow injection and exponential dilution system. Air (20.95% v/v O2 ) sample at volumes of (1) 0.2; (2) 0.3; (3) 0.4; (4) 0.5 ml were injected into the system. Table 1 Linear regression analysis for oxygen standards employing flow injection and exponential dilution system Air injection volume (ml)
Slope (%−1 )
Y-intercept
r2
n
0.2 0.3 0.4 0.5
12.06 ± 0.09 12.18 ± 0.08 11.84 ± 0.06 11.64 ± 0.05
−0.00654 ± 0.00110 −0.00702 ± 0.00136 −0.01146 ± 0.00142 −0.01078 ± 0.00134
0.9990 0.9992 0.9994 0.9995
21 23 25 28
ronment the Stern–Volmer plots of the O2 quenching data for luminescence transition metal complexes will turn nonlinear at high concentrations of O2 [8,12,15]. The nonlinearities of the Stern–Volmer plots can be attributed to the existence of inhomogeneities in the binding sites in the silica gel leading to the differences in the local O2 quenching constants. The curvature of the Stern–Volmer plot is a typical characteristic of multiple species. In general, a two-site quenching model is sufficient to describe the quenching behaviour of O2 on the ruthenium complex immobilized on the substrate. The two-site Stern–Volmer equation [8,47,48] is
I0 = I
f1 1 + kSV1 [O2 ]
+
f2 1 + kSV2 [O2 ]
−1 (3)
where I0 and I are the luminescence intensities in the absence and presence of O2 , respectively, [O2 ] is the O2 concentration, f1 and f2 are the fractions of the quenching sites 1 and 2, respectively; kSV1 and kSV2 are the Stern–Volmer quenching constants for sites 1 and 2, respectively. This nonlinear two-site Stern–Volmer equation was then employed to fit the data sets obtained from the continuous flow and flow injection methods so as to cover a wide range of O2 standards from low to high concentrations. Fig. 6 dis-
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Fig. 6. The calibration plot for the oxygen-sensitive material at various oxygen contents using the two-site quenching model. Stern–Volmer plot of (I0 /I) versus O2 concentration. The inset diagram shows the linear regression plot at low O2 concentration ranges (0.0–0.056% v/v).
Table 2 Two-site fitting parameters for quenching of the oxygen-sensitive material O2 concentration range (%)
f1
f2
kSV1
kSV2
r2
n
0.0−0.55
0.54 ± 0.08
0.46 ± 0.08
4.94 ± 0.53
22.46 ± 4.05
0.9997
36
plays the two-site Stern–Volmer plot, and the fitting parameters for the two-site model are also given in Table 2. The data sets fit extremely well to Eq. (3). The fractions of quenching sites 1 and 2 are approximately equal but their quenching efficiencies are completely different. Quenching site 2 is about 4.5 times more efficient than quenching site 1. The inset diagram shown in Fig. 6 indicates that the calibration graph is linear at low O2 concentrations (0.0–0.056% v/v). At low O2 concentrations Eq. (3) is simply converted to Eq. (1) according to the fitting parameters for the two-site model (Table 2). The limit of detection calculated from Eq. (1) as the O2 concentration which produced an analytical signal equal to three times the standard deviation of (I0 /I) − 1 at zero value was found to be 3.6 ppm (v/v).
3.4. Reversibility and response time Fig. 7 shows the typical response of the O2 -sensitive material towards step changes of various O2 concentrations. It demonstrates that the sensor can continuously monitor the O2 content. The response is fully reversible and no hysteresis is found. The t95 response times of the sensor are <0.2 s on going from N2 to 1.0% O2 and <1 s on going from 1.0% O2 to N2 . 3.5. Photostability and long-term stability The photostability of the O2 -sensitive material is extremely good. There was no sign of photodegradation when it was irradiated at 468 nm using an xenon
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Fig. 7. Typical response curves of oxygen-sensitive material at excitation/emission wavelengths of 468/604 nm on exposure to various O2 contents (0.0–0.55% v/v) using the continuous injection mode.
lamp which was set at 70 W for 12 h. The fluorescence intensity was stable throughout 12 h excitation light exposure. The long-term stability of the optical sensor is also good since the analytical performance of the optical sensor did not show any change over 6 months storage in the dry box.
4. Conclusions A single standard calibration method has been successfully applied to calibrate the O2 -sensitive material tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) ditetrakis(4-chlorophenyl)borate-adsorbed silica gel which is capable of monitoring O2 content continuously. The ruthenium(II) complex-adsorbed silica gel possesses high photochemical stability and is not leached out by washing with organic solvents [49] or aqueous solutions. The sensor has high photostability, no hysteresis and long lifetime. Last but not the least the optical sensor can also be easily adapted for
remote measurements by using fibre optics in future developments.
Acknowledgements Financial support of this research from HKBU (project nos.: FRG/96-97/II-54 and FRG/96-97/II-62) is gratefully acknowledged.
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